Prime Minister Gordon Brown may have to pay back some of his expense claims, the BBC understands.

The BBC's political correspondent Iain Watson said it is "highly likely" that Mr Brown will have to pay back cash.

Whitehall sources have indicated "the majority" of MPs will either have to justify their claims or pay money back.

Several MPs announced that they would stand down from office, after the publication of hundreds of claims earlier this year.

Mr Brown is among possibly hundreds of MPs due to receive letters this week from the auditor appointed to investigate expense claims.

Former senior civil servant Sir Thomas Legg is understood to be writing to the MPs asking them to either give more details on their expense claims, or pay the money back into the public purse.

IAIN WATSON, POLITICAL CORRESPONDENT
Iain WatsonThe Prime Minister is likely to become a victim of his own clean up campaign.

Back in May, facing difficult European elctions - and a welter of expense allegations in the Telegraph, it was Gordon Brown himself who called for all claims on MPs' second homes to be independently audited.

Now that they have been, it's highly likely he too may have to stump up some cash. Indeed he's already paid back Â£153 for wrongly claiming twice for a plumbing bill. Any further repayments may be modest but the political damage is likely to be much greater.

Although there is no suggestion of impropriety on his part -and even though his opposite number David Cameron has already paid back nearly Â£1,000 in expenses himself - polling suggests that voters place more blame on the government than the opposition.

Last May it emerged that Mr Brown had paid his brother Andrew Â£6,577 for arranging cleaning services for his Westminster flat for 26 months.

At the time, Downing Street said the brothers had shared a cleaner who worked in both their flats. Andrew Brown had paid her wages and had then been reimbursed by the prime minister, who then claimed cleaning expenses.

It was also revealed the prime minister had claimed twice for the same plumbing work within six months of each other. The House of Commons Fees Office said this had been an "inadvertent mistake".

It apologised for having not spotted it and Mr Brown is understood to have repaid the sum involved - believed to be Â£150.

It was also reported that Mr Brown may have billed taxpayers for his Sky Sports subscription.

On Saturday Mr Brown told a newspaper that "the worst offenders" among MPs who claimed bogus expenses should be prosecuted.

'Switching' homes

After the claims were made public in the Daily Telegraph earlier this year, many MPs were accused of extravagance, over-claiming and avoiding tax on home sales.

Several were found to have repeatedly "switched" their designated second homes, meaning they were able to refurbish both their homes at public expense.

The BBC understands that Sir Thomas - who was appointed by Downing Street - has been looking at whether MPs were using money to improve their properties, rather than just maintain them as the rules allow.

He is also thought to have examined instances where MPs used public money to pay off the capital on their mortgages, rather than mortgage interest.

WITHIN THE RULES
Â£24,000-a-year Additional Costs Allowance, which covers the running of MPs' second homes
Â£22,193-a-year Incidental Expenses Provision, which pays for running an office
Â£10,400-a-year Communications Allowance, which funds websites, newsletters, stationery and postage

BBC political correspondent Iain Watson says Sir Thomas will allow MPs to make "fair representations" if their claims are challenged - but once that process is complete, his full report will be made public, probably in December.

International Development Secretary Douglas Alexander told the BBC that MPs had a duty to respond to the letters.

"I'll wait for my letter like every other member of the House of Commons.

"This is the next stage in a process. There's been an independent audit of everybody's claims, I think, over the past five years.

"And so I expect on Monday... we'll all receive a letter and if there are questions to be answered then I hope every member of Parliament answers them."

The Sunday Telegraph reported 325 MPs could be facing further queries about their expense claims, although this figure could not be verified independently.